Since I restore vintage bikes as a hobby, I made several gas tanks using the POR-15 kit. Along the way I learned a lot about what to do and what to avoid when using the kit.
I was asked to post my procedure and tips on another forum and the members found it very helpful. So I'm going to post it here in the hope that it will help others as well.
First, let me say that I have used a few different gas tank stripper/liner kits, including Red-Kote, but I have found the POR-15 kit to be the best and most comprehensive of all.
Here is a link to the kit on their website:Motorcycle Fuel Tank Repair Kit POR-15
HERE ARE MY APPROACHES AND TIPS:
The first thing I'm going to say is determine how extensive your tank rack is. If only some spots on the tank are rusted (usually the bottom), the second step of the POR-15 kit (metal prep solution) is enough to remove the rust. However, if you have a tank, as I have once or twice, that is almost completely rusted on the inside, you will need to de-rust that tank with something else before starting the POR-15 kit. This kit, and most others, do not have enough rust remover to deal with that much rust.
The best and safest rust remover I have ever used is "EvapoRust". It's non-caustic and you can safely pour it down the drain when you're done. It can be reused until it is very dark in color, almost black, which means that it no longer holds up to rust. Not all auto parts stores carry it. O'Reilly's is the only one around here that sells gallon jugs. It is expensive, so buying it by the liter is not the way to go. A gallon costs over $20.00. But is it worth it. The stuff is amazing. I usually buy enough to fill half the tank. I pour about two gallons into the tank and let it sit in various positions for at least several hours. Back Down - Forward Down - Down Down - Left Side Down - Right Side Down - Up Down. That means you'll probably have the stuff in there for two full days. This will remove 99% of the rust.
Do not rinse the EvapoRust from the tank with water until you are ready to start the POR-15, otherwise the tank will rust quickly. After rinsing with plenty of clean water, you can start using the POR-15 kit.
The POR-15 kit actually comes with an excellent guide. They go out of their way to explain exactly what to do and more importantly what not to do! I will expand them here.
Note that I typically plug round holes, like the gas cap hole, with Oatley test plugs from the plumbing section of a hardware store. Just get the proper size cap that fits the filler hole in your gas cap.
The first stage of the POR-15 kit is the Marine Clean. This is mixed with a liter of warm water and then goes into the tank and is manually poured for 20 minutes and then rinsed with water. Marine Clean removes gum and varnish deposits from old fuel that was in the tank.
The second stage is the preparation of the metal. This is POR-15 rust remover. It also prepares the metal for sealing by making it acidic. That goes into the tank and is manually poured over 20 minutes. After splashing around, you should again let the tank sit in various positions for about 15 to 20 minutes at a time, allowing it to work on each internal surface of the tank to remove any remaining rust. The instructions for POR-15 state not to leave it in the tank for more than 2 hours total. Keep in mind that this is only a quart that you put into a 3 to 5 gallon gas tank, depending on the model. Therefore, it should be positioned so that the material touches all tank surfaces for 15 to 20 minutes. Then you need to rinse things with lukewarm water, not cold water from a hose. It must be rinsed MULTIPLE times to ensure that all of the Metal Prep solution comes out.Now the most important step that can mess things up if not done right.
The tank should now be COMPLETELY DRY before pouring the sealant. There will be places in the tank that will hold water, but you have to spend time getting the tank completely dry on the inside. The recommended drying method is to open any clogged holes in the tank and use a hair dryer or heat gun to blow hot air into the gas tank filler hole. If you decide to use a heat gun, be careful, you can melt paint right out of your tank if you're not careful. It is best to use a hair dryer for this. Also, I learned how to do these outdoor tank restorations on a hot sunny day. The heat of the sun is of great help in this drying process. Between sessions of blowing hot air into the tank with a blow dryer, I let the gas tank bake in the sun for 10-15 minutes each. TIP: If after about a half hour between blow drying and sunbaking, the tank is so hot that you can't lift it with your bare hands for more than a few seconds, you can be pretty sure it's completely dry on the inside. This is how I measure drying success. If it's not completely dry and you pour tank sealer it won't stick to metal and can shine anywhere! (You don't want that because that sealant is nasty, like liquid metal, and doesn't clean up well.)
NOTE: Once the tank is completely dry, you will notice that flash rust (a slightly reddish-brown color on the metal surface) has already formed inside the tank. Don't worry, this is normal and there is nothing you can do about it. Tank sealant adheres to and seals the tank with a rust film. Just don't wait too long after the tank is dry to begin the tank sealing process.
The last stage is the tank sealer. You need to start as soon as the tank is completely dry, as rust will form. As mentioned, this is like liquid metal and hard to separate from things like your fingers and the gas tank. Wear a pair of rubber gloves. If you put it in your tank and don't clean it right away, you're in trouble. If you get something on your fingers, wipe it off, otherwise it will stain everything you touch. It is best to cover at least the top of your tank with something 6" to 8" around the fill hole. I use painter's tape. It holds up well enough to keep the goo out, but not so tight that you'll strip paint off a very old tank. Do not use tape on the painted surfaces of your tank! I used duct tape to remove paint from an old tank before! Use an old funnel to pour the sealer into your tank. If you miss it and spill something in the tank, you'll get mad. Use an old funnel that you can throw away when you're done, or make a funnel like I did out of paper and then throw it away. Now close the filling opening and make the SLOW ROLL. You must slowly roll the tank in all possible positions so that the seal covers the entire interior of the tank. SLOW roll is the operative term. The material is as thick as molasses, so if you roll too fast, you can't keep up. It rolls nice and slow. I usually shoot for about 10-15 minutes total.The last major step is to drain any remaining sealant from the tank.
When you have an excess, it usually builds up deep in the tank, and that's a bad thing. It may refuse to harden and block the flow of fuel. You can usually drain it out of the petcock hole. This usually works. Sometimes you just can't get everything out of the petcock hole, but you can still see some pooling through the gas tank filler hole if you tilt the tank in the right direction. On one or two occasions I had to use a few small pieces of paper towels (about 4" x 4") inserted into the tank with a flexible long-necked tong to soak up some of the excess fuel that wouldn't come out of the petcock. Hole. If you do this, be sure to use heavy-duty paper towels, not the flimsy white ones from your kitchen. These could tear and then fish pieces of paper out of your tank. Also, be careful not to get sealant on the tank when you remove the paper towels from the fill hole. Have something handy to drop the caulk-coated paper towels on when you take them out. Like I said, things are hard to clean once they're on.
Be sure to rinse the sealant out of your Oatley test plug as soon as you remove it from the tank or you may never use it again. Also, don't be tempted to rinse the caulk down your sink at home! This stuff dries as hard as metal and you don't want it flushing down your house drain. Mom will not be happy! Do all your outdoor showers with a hose.
Now the tank needs to be hardened.
before you can refuel. Do not change the curing time. At least 4 days! Placing it somewhere exposed to direct sunlight will help the curing process. Any sealed holes must be open to heal.
Well... these are my lessons learned from various tanks with the POR-15 kit. I hope this helps.